The Educationalist | Question 10 | May/June 2026 Physical Science Paper 1

[00:00:00]Right, and now we look at question number 10.

[00:00:04]Right, they say to us we've got an experiment and in an experiment a metal is identified using a using the photoelectric effect.

[00:00:14]So, the photons of light with different energies are shown onto the metal surface and the corresponding maximum velocities of the ejected electrons are recorded.

[00:00:27]They say the graph below shows the relationship between the square of the maximum velocities of the ejected electrons and the energy of the incident photons. Okay, they say the value of X is unknown.

[00:00:42]Right.

[00:00:44]So, they say first define the term work function. You know that's the minimum energy required to emit electrons from a metal surface, right?

[00:00:55]So, the one thing that I want to point out to you in relation to this question is that when you look at it, um, please note I know usually we used to having frequency over here um, against uh, kinetic energy. But, in this case what they did was to give us the energy of the incident photons, okay? And against this is the maximum speed the square of the maximum speed of the electrons. Now, they say to us one of the metals in the table below is used in this experiment. Use a suitable calculation to identify the metal used.

[00:01:38]Right, now it is very important. So, they've given us the work function of each of the metals. So, we need to be able to calculate the work function and compare uh, or at least, um, determine which one uh of the metals they used. So, we can only do so by calculating work function. So, I'm going to say well, E is equal to omega zero, work function, plus E K max.

[00:02:12]Right?

[00:02:13]Now, what did they give us in relation to the kinetic energy?

[00:02:19]Or rather, uh not kinetic energy, rather uh with regard to the energy of the photon. They gave us the energy of the incident photons, right?

[00:02:29]So, I'll choose a value that you know there.

[00:02:33]So, that's 7 * 10 to the exponent minus 19.

[00:02:38]So, we've got this guy.

[00:02:44]Right. So, uh my device is complaining already.

[00:02:49]Okay, let's charge it quickly.

[00:02:51]So, 7 * 10 to the power -19.

[00:02:55]We want the work function, but please, ladies and gents, remember how do I get E K max? That's half M V squared, right?

[00:03:05]That's half times M.

[00:03:07]That's V max squared.

[00:03:10]So, that's 7 * 10 to the power -19.

[00:03:15]We're looking for the work function.

[00:03:17]But remember, you are given the mass of the electron, okay? Right, remember that this is the mass of the electron, it's given to you.

[00:03:29]That's 9.11 * 10 to the power -31.

[00:03:36]And we've got the maximum square velocity when our energy is 7, and the maximum velocity squared of the electron is 6.59, Okay.

[00:03:54]times 10 to the power 11.

[00:03:58]So, that's 6.59 times 10 to the exponent 11. Now, please note that is already squared. So, please do not square it. Okay? So, all I'm going to do is take this all of this to the other side.

[00:04:16]All right? And of course it I will subtract it.

[00:04:19]And Yeah, from that value 7 times 10 to the power -19.

[00:04:27]Okay, let's do that.

[00:04:31]-19 And subtract 0.5 times 9.

[00:04:40]11 >> [clears throat] >> exponent -31 multiplied by 6.59 exponent 11.

[00:04:59]Right, and I get 3.998, which is approximately 4.

[00:05:04]Okay? And our work function is 4 times 10 to the power -19 joules. And ladies and gents, you can see that our metal of first day is barium. Okay?

[00:05:24]Right, and then they say to us calculate X.

[00:05:30]So, now that we are looking for X, we now know what the work function is, right?

[00:05:37]We're looking for Vmax squared.

[00:05:42]So, I'm going to say E is omega zero plus EK max.

[00:05:53]Right, we know that when the energy of the photon was 5.4 times 10 to the power minus 19, we know our work function is four times 10 minus 19.

[00:06:10]This is plus half >> [snorts] >> of 9.11 times 10 to the power minus 31.

[00:06:21]And this is V max squared. We are looking for V max squared, so I'm going to take the difference between the two and divide by this entire thing.

[00:06:35]Right, so I'm going to say 5.4 exponent minus 19 minus four times exponent minus 19.

[00:06:49]Okay, get a difference between the two and I divide by 0.5 times 9.11 exponent minus 31.

[00:07:04]Okay.

[00:07:06]Right, so we get that value over there.

[00:07:09]Let's try and get it in scientific form.

[00:07:13]Okay, remember we are looking for the V max squared, so V max squared will be equal to it's 3.07 times 10 to the exponent of 11.

[00:07:33]And remember this is going to be meter squared per second squared. So, meaning X because we already have the times 10 to the exponent 11, means the value of x is 3.07.

[00:07:52]Okay.

[00:07:53]Right. And so, that is how the cookie crumbles. All right, let's go to the last question.

[00:07:59]So, they say to us, "How will the maximum velocities of the ejected electrons change when the intensity of the incident light is increased?"

[00:08:09]Now, remember when they tell us about intensity, they are simply referring uh to not the energy, rather, but the brightness of light, okay? So, they are increasing the brightness, and they want to know how will the velocities be affected.

[00:08:31]Now, remember that the only thing that can affect velocity is the frequency of light, and therefore, which will affect the energy of the incident light, right?

[00:08:41]So, in that case, means that they the energy will remain the same.

[00:08:47]And they say, "Give a reason for your answer."

[00:08:49]Um in this case, intensity, or rather, changing intensity will not increase the energy of the incident light, or rather, will not change will not change the frequency of the incident light.

[00:09:27]Okay?

[00:09:28]And if the energy does not change, the the kinetic energy will also, because work function, rather, remains constant.

[00:09:36]Um if energy of the incident light does not change, then neither will the kinetic energy. Okay?

[00:09:44]Right, ladies and gents, that was out of 150 marks.

[00:09:49]I'll say this um as I conclude on what we've just done.

[00:09:54]Please, I want you guys to always always keep in mind. You know, it's important for you uh as I said, you know, once you you get to that point where uh you've practiced uh using past exam uh exam papers, right?

[00:10:10]It's always important to also be mindful. Um, I would I would recommend, okay, maybe let me finish my thought.

[00:10:17]I would also want you to be mindful to go through the theory, uh go through your you know, your definitions and so on, but it's important to also start recognizing the pattern, right?

[00:10:33]Um, of course, the standard of the exams uh keeps increasing, going higher and higher. Um, and so, even if you are doing DBE, can I then suggest to you that please do look at IEB exams, right? And the same for the IEB people, uh but also look at the Sakai exams. I I I really find them uh quite um helpful, right? In terms of just bridging that gap as well.

[00:11:02]So, uh ladies and gents, that brings us to the end of this question paper. I really hope that my analysis has been worthwhile and that you enjoyed this.

[00:11:13]Otherwise, your favorite uncle will see you next time. Don't forget to subscribe.

[00:11:19]Shap, shap.